Steam and gas turbines are used, among other purposes, to power electric generators. Gas turbines are also used, among other purposes, to propel aircraft and ships. A steam turbine has a steam path which typically includes in serial-flow relation, a steam inlet, a turbine, and a steam outlet. A gas turbine has a gas path which typically includes, in serial-flow relation, an air intake or inlet, a compressor, a combustor, a turbine, and a gas outlet or exhaust diffuser. Compressor and turbine sections include at least one circumferential row of rotating buckets. The free ends or tips of the rotating buckets are surrounded by a stator casing. The base or shank portion of the rotating buckets are flanked on upstream and downstream ends by the inner shrouds of stationary blades disposed respectively upstream and downstream of the moving blades.
The efficiency of the turbine depends in part on the axial clearance or gap between the rotor bucket shank portion angel wing tip(s) (seal plate fins), and a sealing structure of the adjacent stationary assembly, as well as the radial size of the gap between the tip of the rotating buckets and the opposite stationary assembly. If the clearances are too large, excessive valuable cooling air will leak through the gaps between the bucket shank and the inner shroud of the stationary blade and between the tips of the rotating buckets and the stationary assembly, decreasing the turbine's efficiency. If the clearances are too small, the rotating blades will strike the sealing structure of the adjacent or opposite stator portions during certain turbine operating conditions.
In this regard, it is known that there are clearance changes during periods of acceleration or deceleration due to changing centrifugal forces on the buckets, turbine rotor vibration, and/or relative thermal growth between the rotating rotor and the stationary assembly. During periods of differential centrifugal force, rotor vibration, and thermal growth, the clearance changes can result in severe rubbing of, e.g., the moving bucket tips against the stationary seal structures or against the stationary assembly. Increasing the tip to seal clearance gap reduces the damage due to metal-to-metal rubbing, but the increase in clearance results in efficiency loss.
More particularly, during turbine operating conditions the components of the turbine can thermally expand (or contract) at varying rates due to high operating temperatures in excess of 2,000 degrees Fahrenheit. The stator and rotor must be maintained apart from each other across all operating conditions to prevent damage from contact with each other. However, if a single fixed positional relationship between the stator and rotor is maintained across all operating conditions then for at least some operating conditions, i.e., startup, there will be compressed fluid leakage between the stator and rotor assemblies leading to operating inefficiencies.
It is known in the art to facilitate compressor casing movement by using pressure difference in plenums purged with extracted air. It is also known in the art to use a thermally expandable linkage to facilitate compressor casing movement and to use an air driven or stream driven piston to facilitate compressor casing movement.